Valve devices for electropneumatic brake control devices for control of vehicle parking brakes are known, for example, from DE 103 36 611 A1 and EP 1571061 A1. Such known brake control devices are used in brake systems that are provided not only with a service brake, which can be actuated by means of a brake pedal, but also with a parking brake (often referred to as a handbrake), which can be actuated by means of an electrical signal transducer.
In these known brake systems, the parking brake is regularly applied by means of spring-actuated brake cylinders. In order to release the parking brake, the spring-actuator part of the spring-actuated brake cylinders must be pressurized with compressed air. For this purpose, air must be admitted to the spring-actuator part. The necessary pressure for air admission is supplied from a compressed-air reservoir. However, this pressure supply is not permanently turned on, but can be shut off. Furthermore, the pressure in the spring-actuated brake cylinder can also be lowered, and, so, the spring-actuator part can be vented. The parking brake is applied by such venting.
To control the pressure in the spring-actuated brake cylinder, a relay valve is conventionally provided. With this relay valve, the pressure supply from the compressed-air reservoir tank to the spring-actuator part of the spring-actuated brake cylinders can be controlled. Control is exercised with the assistance of electropneumatic valve devices, especially, by means of electrically actuatable solenoid valves, which regulate a control pressure supplied to the relay valve.
In the brake system described in DE 103 36 611 A1, a bistable valve is used for this purpose. This valve can occupy two stable conditions. In the event of a power failure, it maintains the condition set at that instant. Furthermore, a holding valve is connected between the bistable valve and the control input of the relay valve in this brake system. By means of the bistable valve and of the holding valve, the pressure at the control input can be held, raised or lowered. The pressure at the output of the relay valve varies correspondingly. In this way, the parking brake can be released or applied by means of electrical signals to the bistable valve and holding valve.
Because of its construction, however, the bistable valve is complex and, therefore, expensive. Furthermore, as noted above, the bistable valve remains in its previous condition in the event of failure of the electrical power supply. Thus, a vehicle having such a brake system might not be able to be safely parked in the event of failure of the electrical power supply. In other words, it might not be able to be parked by automatic and permanent venting of the spring-actuator part of the spring-actuated brake cylinders—that is, by application of the parking brake.
Electromagnetically actuatable multi-way valves of less complex construction are proposed in DE 35 01 708 A1. In particular, it is proposed that two valves disposed opposite one another be provided in a multi-way valve. An armature that cooperates with a coil disposed between the two valves is associated with each of these two valves. The springs associated with these armatures are designed such that different magnetic forces are necessary for actuation of the two valves. These magnetic forces are generated by application of a current flowing through the coil. Because a separate armature is associated with each valve, mutually independent actuation of the two valves is possible by appropriate energization of the coil.
However, this known valve has the disadvantage that, in de-energized condition, its input is in communication with the output leading to the consuming load, whereas the outlet of the valve is shut off. If such a valve were to be used instead of the aforesaid bistable valve and holding valve, the full reservoir pressure would be injected into the control input of the relay valve in the event of failure of the electrical power supply, causing the parking brake to be released by the resulting admission of air to the spring-actuator part of the spring-actuated brake cylinders. This is undesirable, however, since the vehicle might no longer be safely parked in the event of failure of the electrical power supply.